The present invention relates to material handling equipment and, more particularly, to a conveyor system having a spiral portion.
One type of conveyor is known having a conveyor belt in which a portion of the conveyor belt is formed into an ascending or descending spiral. The ends of the spiral are connected together through an out-run, a return-run and an in-run. The conveyor is at least partly driven by a capstan rotating within the spiral portion and contacting the inner edge of the conveyor belt as it traverses the spiral. Frictional forces are controlled by a sprocket drive on the out-run which controls longitudinal tension in the conveyor belt. In effect, the amount of drive is controlled by the amount of longitudinal tension applied to the out-run. This is similar to a nautical winch in which a plurality of turns in the bight of a rope are wound on a rotating reel. If the rope exiting the reel is left slack, the reel turns with insufficient friction with the rope to pull the incoming rope. If a moderate tension is applied to the exiting rope, a substantial force is applied to the incoming rope.
Spiral conveyors are disclosed in U.S. Pat. No. 3,348,659 (now expired) and 4,078,655.
Spiral conveyors of the type described above, and disclosed in the referenced patents, are frequently used in food-preparation establishments to hold freshly baked goods in a cooling environment for a substantial time prior to a succeeding operation such as, for example, packaging. The food-preparation industry shares with other industries the desire for improved productivity. One approach to improved productivity includes increasing the throughput of spiral conveyors. Throughput is achieved by increasing the linear speed of the conveyor belt.
As the linear speed of the conveyor belt increases, it becomes increasingly difficult to couple enough torque through the frictional contact between the outer surface of the capstan and the inner edge of the belt without applying destructive tension in the conveyor belt. The lack of friction between the belt and the capstan is especially acute if the belt is made of plastic. Such low friction becomes even lower if there under wet and/or icy conditions, which occur very frequently in the food industry.
Making belts from plastics has some distinct advantages, however. Some of these advantages are: the price of a plastic belt is usually lower than that of a metal belt; the weight of the plastic belt is significantly lower than the weight of the metal belt; it is substantially easier to maintain the plastic belt clean due to the lower adhesion of production debris and dirt to plastic. Plastic belts driven by sprocket wheels are described, for instance, in the U.S. Pat. No. 4742907. Plastic belts are also described in the U.S. Pat. No. 4,394,901. However, plastics have lower load bearing capacity than metals, and their use in spiral systems is restricted.
Therefore, one of the problems with conveyor belts for spiral systems is difficulty of the transmittal of a driving force from the capstan to the belt. The difficulty is especially severe in the case of plastic belts.